diff --git a/cpp2rust/converter/converter_lib.cpp b/cpp2rust/converter/converter_lib.cpp
index 3f515788..325fa15a 100644
--- a/cpp2rust/converter/converter_lib.cpp
+++ b/cpp2rust/converter/converter_lib.cpp
@@ -380,7 +380,7 @@ clang::QualType GetReturnTypeOfFunction(const clang::CallExpr *expr) {
   return {};
 }
 
-std::string GetOverloadedOperator(const clang::FunctionDecl *decl) {
+const char *GetOverloadedOperator(const clang::FunctionDecl *decl) {
   switch (decl->getOverloadedOperator()) {
   case clang::OO_Less:
     return "lt";
diff --git a/cpp2rust/converter/converter_lib.h b/cpp2rust/converter/converter_lib.h
index 3696d680..668e5150 100644
--- a/cpp2rust/converter/converter_lib.h
+++ b/cpp2rust/converter/converter_lib.h
@@ -91,7 +91,7 @@ template <class T> llvm::SmallString<16> GetNumAsString(const T &num) {
 
 clang::QualType GetReturnTypeOfFunction(const clang::CallExpr *expr);
 
-std::string GetOverloadedOperator(const clang::FunctionDecl *decl);
+const char *GetOverloadedOperator(const clang::FunctionDecl *decl);
 
 bool IsOverloadedComparisonOperator(const clang::CXXMethodDecl *decl);
 
diff --git a/cpp2rust/converter/mapper.cpp b/cpp2rust/converter/mapper.cpp
index a100c969..e0e84c51 100644
--- a/cpp2rust/converter/mapper.cpp
+++ b/cpp2rust/converter/mapper.cpp
@@ -367,7 +367,7 @@ Map::const_iterator parallel_search(const Map &container,
 
 decltype(exprs_)::const_iterator search(const clang::Expr *expr) {
   auto qualified_name = ToString(expr);
-  auto result = parallel_search(exprs_, [&](std::string tpl) {
+  auto result = parallel_search(exprs_, [&](const std::string &tpl) {
     return matchTemplate(tpl, qualified_name);
   });
   llvm::errs() << "search expr " << qualified_name << ", result:\n";
@@ -382,7 +382,7 @@ decltype(exprs_)::const_iterator search(const clang::Expr *expr) {
 decltype(types_)::const_iterator search(clang::QualType qual_type) {
   auto type = ToString(qual_type);
   auto result = parallel_search(
-      types_, [&](std::string tpl) { return matchTemplate(tpl, type); });
+      types_, [&](const std::string &tpl) { return matchTemplate(tpl, type); });
   llvm::errs() << "search type " << type << ", result: "
                << ((result == types_.end()) ? "None"
                                             : result->second.type_info.type)
@@ -531,8 +531,9 @@ clang::QualType normalizeQualType(clang::QualType qual_type) {
 }
 
 std::string mapTypeStringRecursive(const std::string &cpp_type) {
-  auto rule = parallel_search(
-      types_, [&](std::string tpl) { return matchTemplate(tpl, cpp_type); });
+  auto rule = parallel_search(types_, [&](const std::string &tpl) {
+    return matchTemplate(tpl, cpp_type);
+  });
   if (rule == types_.end()) {
     llvm::errs() << "cpp_type: " << cpp_type << '\n';
     assert(0 && "Type is not present in types_");
diff --git a/cpp2rust/converter/models/converter_refcount.h b/cpp2rust/converter/models/converter_refcount.h
index 81025901..a3479480 100644
--- a/cpp2rust/converter/models/converter_refcount.h
+++ b/cpp2rust/converter/models/converter_refcount.h
@@ -196,7 +196,7 @@ class ConverterRefCount final : public Converter {
     FullRefCount,
   };
 
-  std::string ConversionKindToString(ConversionKind k) {
+  const char *ConversionKindToString(ConversionKind k) {
     switch (k) {
     case ConversionKind::Unboxed:
       return "Unboxed";